Optical time domain reflectometry (OTDR) is widely used in the field of telecommunications. In OTDR, a short pulse of light (typically, laser light of duration between 10 ns to 10 μs) is launched into an optical fiber (the fiber under test, or FUT). Reflected light returning from the fiber is collected by a photodetector (e.g., a photodiode), and displayed. The OTDR further notes the difference between the launch time and the detection time. Using OTDR, a spatially resolved analysis of the backscattered signal over the entire length of the FUT is obtained. By reviewing the recorded amplitude versus time trace, a technician may identify breaks, damage, strong reflections, bad connections, strong bends, crushing, and other characteristics of the FUT.
For simplicity, the backscattered signals that are captured and measured by the OTDR are referred to as an “RBS signal” even though the signal is actually made up of multiple distinct signals. The RBS signal measured by the OTDR occurs due to microscopic fluctuations or defects in the fiber, which cause the light launched into the FUT to scatter in all directions. Part of this scattered light, the RBS signal, is coupled back in the backward direction of the fiber, and may thus be measured by the OTDR. The intensity of the RBS signal is proportional to the duration of the incoming light signal, which is typically a short width laser pulse. Since the backscattered signal strength depends on the losses in the FUT, the attenuation of an optical signal propagating through the fiber may be measured as a function of the distance.
With very short laser pulses being launched into the FUT, the OTDR is able to achieve a high temporal/spatial precision. Since the RBS signal is proportional to the width of the laser pulse, the shorter the laser pulse width, the less optical power to be received at the photodetector of the OTDR.
On the other hand, to increase the resolution of the OTDR, the photodetector (and associated amplifiers) needs to have a larger bandwidth. Such high-bandwidth devices have a lower sensitivity. For this reason, conventional OTDRs are unable to simultaneously achieve high spatial/temporal resolution and high sensitivity.
A photodetector that performs photon counting may overcome these constraints. Photon counting allows detecting very low light levels, down to the single photon level. Photon counting techniques may achieve a temporal resolution of better than 1 nanosecond (ns). However, photodetectors that perform photon counting have a limited dynamic range. The dynamic range of an OTDR is the difference between the highest and the lowest measurable RBS signal. Since a photon-counting OTDR cannot detect more than one photon for each emitted laser pulse, high backscatter levels lead to a saturation of the photodetector.
Thus, there is a continuing need for an OTDR that simultaneously achieves a high temporal/spatial resolution and a high dynamic range.